This invention relates to a digital communication system, and more particularly to a digital communication system using xcfx80/4 shift QPSK modulation system for mobile digital communications.
In conventional mobile communication systems, lightening of terminal device and minimization of volume have been key factors. To realize the lightening of terminal device and the minimization. of volume, in particular, the minimization of battery volume is an important problem. Along with this problem, xcfx80/4 shift QPSK modulation system with low consumed power has been widely used.
In a conventional PSK modulation system (eight-value PSK system), as shown in FIG. 1, signal allocation points (shown by a circle in FIG. 1) that can be shift to at the next time are eight, and 3-bit information from 000 to 111 is assigned by those signals and a reference signal (signal of one timing before) shown by an arrow in FIG. 1.
On the other hand, comparing with the PSK modulation system, in the xcfx80/4 shift QPSK modulation system, as shown in FIG. 2, eight signal allocation points are divided into two groups. When a signal exists on any point of one group of signal allocation points (shown by a circle in FIG. 2) at certain timing, the signal can shift to any point of another group of signal allocation points (shown by a triangle in FIG. 2) at the next timing. Therefore, the amount of information transmittable is four (2-bit) whereas the signal allocation points is eight (3-bit). Meanwhile, in FIG. 2. arrows indicate a reference signal (signal of one timing before).
In the xcfx80/4 shift QPSK modulation system, the amplitude variation of signal is small and the deterioration and spectrum dispersion on non-linear transmission line is small because the spectrum of modulated wave is phase-shifting without passing the zero point on the signal diagram.
Therefore, this is a very excellent modulation system for mobile communication terminals that have many limitations as to weight, volume, battery capacity, anti-adjacent-channel-interference characteristic etc.
However, in the xcfx80/4 shift QPSK modulation system, there is the demerit that the amount of information transmitted, 22 (2-bit), is small by one bit, as compared with the number of signal allocation points, which is 23 (3-bit)
Accordingly, it is an object of the invention to provide a digital communication system that the transmission capacity is increased.
According to the invention, a digital communication system for mobile communication, comprises:
two independent transmitting systems;
wherein the digital communication system transmits carrying two xcfx80/4 shift QPSK modulated waves on a same frequency while preventing the dispersion of spectrum.
According to another aspect of the invention, a digital communication system for mobile communication, comprises:
means for mixing and outputting outputs from two independent and equivalently synchronized transmitting circuits in xcfx80/4 shift QPSK modulation system: and
means for receiving a mixed wave transmitted from the transmitting circuits and demodulating the received signal by using information as to its phase and amplitude.
According to another aspect of the invention, a digital communication system for mobile communication, comprises the steps of:
mixing signals in xcfx80/4 shift QPSK modulation system to be output from two independent and equivalently synchronized transmitting systems on a same frequency;
transmitting the mixed signal as an output signal; and
demodulating the mixed signal into a bit string from information as to its phase and amplitude;
wherein signal allocation points which are not used in xcfx80/4 shift QPSK modulation system is used by mixing the two independent and equivalently synchronized xcfx80/4 shift QPSK modulated signals.